Auto hammers are commonly used tools in the fields of decoration and fitment. For example, the Chinese patent invention No. 200820161342.1 discloses an auto hammer, which comprises a housing and a nozzle portion being connected to the housing. The nozzle portion is generally composed of a hollow cylindrical sleeve, and a hole within which the magnet is arranged is drilled in the sleeve. The magnet is engaged in the hole so as to attract the nail placed in the striking device, so that the nail can be clamped. The existent defects of this arrangement are that: the magnet is located at the edge of the sleeve so that the nail could not be located at the center of the sleeve and parallel to the center line of the sleeve (namely the nail is skew after being attracted), moreover, the magnet could not clamp other nonmagnetic materials such as tenons and the like.